How to write a function in PHP 5

A function is a block of statements that can be used repeatedly in a program.

A function will not execute immediately when a page loads.
A function will be executed by a call to the function.

Syntax

function functionName() {
    code to be executed;
}

Please watch below video

How to Rewrite Magento Customer Account Navigation Block in your custom Module

It is very simple.

Copy the core/Mage/Customer/Account/Navigation.php to
your local custom module
local/YourNamespace/Customer/Account/Navigation.php which extends Mage core class

write the rule to rewrite block, goto local/YourNamespace/Customer/etc/config.xml

Add lines as shown in video which shows all details.